Interesting question - I've just been pondering that meself and doing some measuring.
Disclaimer first: I've got no certification of any sort in ergonomics, so this is "IMHO" and "YMMV".
Claimer: I'm a career-long computer worker who has spent much time learningthrough medical and physio treatments for repetitive straing injuries - I'm educated and can read and the ergonomics of sewing/quilting joined my topics of interest beside computer ergonomics when I got serious.
The "rules" for a good sewing station are similar to those for a computing station. Ideally, start with your chair height. Sit on chair, adjust so your feet are flat on the floor, and your knees are at a 90 degree angle.
Your desk/sewing table *height* is next, and should be based on this: sitting in your properly adjusted chair, sit up straight, relax your shoulders, and bend your arms 90 degrees at the elbows. Elbows in this position rest directly at your sides, approximately at waist height. The correct table height is actually slightly LOWER than your flat palms in this position. At least for keyboarding, because a typical keyboard will have its key surfaces about an inch above the table's surface, and your 90 deg. angle should bring your palms to "floating" slightly above the tops of the keys, so that if you relax your hands so your fingers droop to a neutral position, they droop to touch they key surfaces. In keyboarding, your work surface is actually the surface of the keys. It is considered acceptable for your work surface to be slightly lower than this, eg, your elbows will be at an angle of slightly more than 90 degrees if this is your comfort/preference. If using an angled keyboard or keyboard tray, the angle should be tilting down, away from you - that is, the number keys, top row of keyboard, are lower than the space bar. Most keyboards and desks to that wrong, even ergonomic ones - they are angled to hyperextend the wrists rather than accomodate relaxed flexion.
Position: Sit in the center of the alpha keys on the keyboard, so each hand has to move an equal distance toward the body's centreline to be in a touch-typing position.
OK - so the above is based on "what I know" - eg, stuff that is documented ergonomic advice and or that docs/physios have told me. After this point, I am extrapolating to my own knowledge of sewing systems and what best works for me in sewing.
Chair mechanics: same. Invest in a good chair. Arms might be less advised for a sewist than a computer geek (more arm movement, particularly if free-motion quilting).
Table: incorporating the above with respect to basic table position, realize that your work surface level is now the top of your fabric. This means the machine plate is essentially your work surface (given fabric thicknesses in millimetres). If you have a sewing station where the machine bed sits flush with the table surface, this is your table height, if your machine sits up on top of the table, you actually need a pretty LOW table, applying ergonomic "rules". To me this means my ideal sewing table is an inch or two higher than my keyboard tray/table but has the cutout of course. (It is still below the height of most non-adjustable office desks).
Position: everyone who has ever mentioned this to me of course says to sit centred in front of the needle.
Distance from edge: I suspect from the other comments this is the most variable and subject to personal preference. I'll ask a physiotherapist if I ever have another chance but I suspect also that I won't get a clear opinion. I'm currently amusing co-workers with some strange fiddling at my desk, but here's how I've just figured out where I seem to like my needle:
Sit in your adjusted chair, at your proper-height tabled, and scoot up as close to the table as is just barely a "natural" seated postion. EG, tummy right up to the edge but don't "squish" any ample parts into the table or have to work to hold yourself that close. Rest your forearms and hands, palms down, on the table surface and double-check that 90 degrees or slightly more at the elbows, with shoulders relaxed and hence elbows staying right at your sides and off the table. In this position if I drew a line between my index fingers, my needle would come down on that line, or slightly on the near side of of it (closer to the edge).
I figure this is approximately right for me, because I will not usally work with belly RIGHT up to the edge of the table, but that is offset by the fact that I DO usually work with my hands' neutral position being slightly in front of the needle as they control fabric feeding in So to actually touch the needle from my natural "working" position requires a slight reach forward with the hands/arms, but NOT so much of a reach that my body must lean. Hope that description makes sense. For me, my absolute maximum measurement would be needle 9.5 inches from the edge, and I think I would actually place it between 7 and 8 inches from the edge.
Because you can always "back up" from the table, but cannot get closer due to the physical barrier of the table edge, I would err on the near side. The only problem I could forsee with putting my needle closer to the edge, say, 4-5 inches, is that when sitting up straight in my proper chair, the angle would be such that I couldn't SEE the needle. This would lead to me curving my back and neck down and possibly tilting my head to the side to get a view of my needle. If you are very short, that's not a problem. If you are very tall it could be a BIG problem. Since you've got the luxury of custom design, try out the measurement/placement and "fiddle" with it, then measure exactly what works for you!
Wow, that got long. Sorry! Can you tell I've spent some time on this very topic?
Hope it is of some use to you, or someone else.